It is a well-known fact that functional-group-terminated polymers alone or in combination with suitable curing agents can provide cured products having high heat resistance and high durability by crosslinking. Representative examples of such polymers are polymers having alkenyl groups, hydroxy groups, or crosslinkable silyl groups at the termini.
Alkenyl-terminated polymers can be crosslinked by photoreaction or in the presence of hydrosilyl-containing compounds functioning as curing agents. Hydroxyl-terminated polymers are cured by the formation of urethane crosslinks resulting from the reaction with polyisocyanates. Polymers having crosslinkable silyl groups at the termini are cured by absorption of moisture in the presence of suitable condensation catalysts.
Main chain skeletons of the polymers having alkenyl groups, hydroxyl groups, or crosslinkable silyl groups at the termini consist of, for example, polymers prepared by ionic polymerization or condensation polymerization. Examples of such main chain polymers include polyether polymers such as polyethylene oxide, polypropylene oxide, and polytetramethylene oxide; hydrocarbon polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, and hydrogenated products thereof; and polyester polymers such as polyethylene terephthalate, polybutylene terephthalate, and polycaprolactone. These polymers are used in various applications depending on the main chain structure and the type of crosslinks. On the other hand, vinyl polymers prepared by radical polymerization and having functional groups at the termini are not currently put to practical application.
Among vinyl polymers, (meth)acrylic polymers have particularly high weather resistance and high transparency, neither of which can be achieved by polyether polymers, hydrocarbon polymers, or polyester polymers. Thus, (meth)acrylic polymers having alkenyl groups or crosslinkable silyl groups in the side chains are applied to weather-resistant paints and the like. Vinyl polymers having alkenyl groups or crosslinkable silyl groups at the termini instead of in the side chain can provide cured products having superior physical properties. Thus, many researchers have investigated simplified methods for producing vinyl polymers having terminal functional groups.
However, in the preparation of vinyl polymers, inhibition of the side reaction is difficult; thus, polymers having a target molecular weight and a uniform molecular weight distribution cannot be readily produced. Moreover, introduction of functional groups to specific positions is particularly difficult. Methods for industrially producing vinyl polymers having terminal functional groups have not been reported so far.
For example, Japanese Unexamined Patent Application Publication No. 5-255415 teaches a method for producing a (meth)acrylic polymer having alkenyl groups at both termini by the reaction with an alkenyl-containing disulfide functioning as a chain transfer agent. Japanese Unexamined Patent Application Publication No. 5-262808 teaches a method for producing a (meth)acrylic polymer, including the steps of preparing a (meth)acrylic polymer having hydroxyl groups at both termini by the reaction with a hydroxyl-containing disulfide, and introducing the alkenyl groups into both termini by utilizing the reactivity of the hydroxyl groups. However, reliable introduction of alkenyl groups into the both termini is not easy according to these methods. In order to reliably introduce functional groups to both termini, a large quantity of a chain transfer agent must be used, and this poses a problem in the manufacturing process.
Furthermore, the present inventors have discovered a method for introducing a functional group into a terminus of a polymer, including the step of polymerizing a vinyl polymer by the living radical polymerization described below; adding a compound containing a functional group and an alkenyl group having a low polymerizability, the compound functioning as an agent for introducing the functional group; and allowing the alkenyl group to react with a terminus of the polymer.
However, this method also has a possibility of not being able to reliably introduce only one functional group into a terminus of the polymer. This is attributable to the following two reasons: One is a decrease in catalytic activity. Depending on the type and/or the quantity of the agent for introducing the functional group, the polarity of the system may change as a result of the addition of the agent, thereby decreasing the catalytic activity. The other is the introduction of a plurality of functional groups due to the presence of the monomer during the step of introducing the functional group. During the step of adding the functional-group-introducing agent, the polymerizable monomer is preferably absent; however, at the end stage of the polymerization, the reaction becomes gradually slower, and trace amounts of polymerizable monomers remain as a result. When the polymerizable monomers are present during the step of adding the functional-group-introducing agent, it is sometimes difficult to control the number of functional groups introduced into one end. After the reaction between the radically propagating terminus and the functional-group-introducing agent, the terminus (having the functional group) normally has low radical reaction activity; thus, it is less likely that the terminus will further react with another molecule of the functional-group-introducing agent. However, when a polymerizable monomer is present in the reaction system during the reaction between the radically propagating terminus and the functional-group-introducing agent, it is possible that the polymerizable monomer having high polymerizability will react with this terminus of the polymer. Once the polymerizable monomer is added to the terminus, the terminus regains high activity and starts to react with another molecule of the functional-group-introducing agent. A plurality of molecules of the functional-group-introducing agent will be introduced in the polymer as a result of the addition of the polymerizable monomer to the terminus and the reaction with the new molecule of the functional-group-introducing agent after the reaction between the radically propagating terminus and the functional-group-introducing agent. When this occurs, it becomes more difficult to introduce one molecule of the functional-group-introducing agent into one terminus of the polymer.
In order to control the rate of introduction of the functional group, the amount of remaining monomer at the end stage of the polymerization may be analyzed so that the functional-group-introducing agent can always be fed at a constant degree of polymerization. However, this requires a complicated process analysis step. According to an approach that does not conduct process analysis, a significantly long time is required before the degree of polymerization reaches a steady state.
When a compound having two alkenyl groups having a low polymerizability is used as the functional-group-introducing agent and the feed amount of the agent is equal to or less than the number of the active termini, both functional groups may react, thereby coupling two polymer termini. In order to reliably introduce the functional groups into the two termini of the polymer, the agent must be charged in an amount larger than that of the propagating termini. In some cases, it is preferable to add an excessive amount of the functional-group-introducing agent in order to increase the reaction rate and to reliably introduce the functional group into the termini. The excess functional-group-introducing agent is recovered after the introduction of the functional group by a process such as distillation under a reduced pressure. These compounds are preferably recycled. In particular, when the functional-group-introducing agent is expensive, recycling of these compounds is particularly important in the manufacturing process.
When a polymerization solvent is used in the polymerization of the vinyl polymer, the polymerization solvent is recovered with the functional-group-introducing agent during the step of recovering the functional-group-introducing agent by reduced-pressure distillation. The polymerization solvent containing the functional-group-introducing agent cannot be recycled as a polymerization solvent since functional groups are introduced into the main chain before a predetermined molecular weight is reached. In order to recycle the solvent as a polymerization solvent, the polymerization solvent must be isolated from the functional-group-introducing agent.
Two or more compounds can be isolated from each other by various techniques, e.g., crystallization and adsorption. Among these, a distillation isolation technique is popular. According to this technique, two or more compounds are isolated from each other based on the difference in boiling point. The isolation is highly difficult when the difference between the boiling points of the compounds is small or when an azeotropic composition is contained. In other words, when the difference between the boiling point of the polymerization solvent and the boiling point of the functional-group-introducing agent is small or when an azeotropic composition is contained, the polymerization solvent and the functional-group-introducing agent can rarely be isolated from each other and thus cannot be recycled as a polymerization solvent and a functional-group-introducing agent, respectively. This poses a problem in the manufacturing process. For example, although 1,7-octadiene (a functional-group-introducing agent) and acetonitrile (a polymerization solvent) have a difference in boiling point of at least 30° C., the present inventors have found that they also contain an azeotropic composition. In such a system, the two components cannot be isolated from each other unless special distillation, such as azeotropic distillation requiring the addition of a third component, is conducted. Furthermore, it is highly difficult to find a suitable third component.
As is described above, the following must be considered in order to reliably introduce one functional group into one terminus of the polymer: a decrease in catalytic activity due to the functional-group-introducing agent; recovery of the functional-group-introducing agent and isolation of the agent from the polymerization solvent; and the timing of addition of the functional-group-introducing agent at a particular degree of polymerization in order to reliably achieve a constant rate of introduction of the functional group.